It is a well known dental technique to evaluate the condition of the tooth pulp by subjecting the tooth to localized stimulation and noting the patient's response. In particular, the tooth may be subjected to electrical stimulation or extremes of heat and cold. Electrical stimulation is typically more convenient, but may be an unsuitable technique under certain circumstances, as for example where a number of teeth are electrically connected by a metallic bridge or similar dental work. Even where electrical testing is available, thermal testing is a useful adjunct, serving as a check on the results of the electrical testing. Moreover, specific known sensitivity to heat and/or cold, as expressed by a patient, can be confirmed by the application of thermal stimulation.
With respect to the localized application of heat or cold, U.S. Pat. No. 3,274,995, to Eidus discloses a dental pulp tester in which a metallic probe tip is thermally coupled to a thermoelectric device utilizing the Peltier effect. The passage of electrical current through the thermoelectric device causes the probe tip to become hot or cold, depending on the polarity of the current. A measure of control over the probe tip temperature is achieved by providing a rheostat to control the current that flows through the thermoelectric device. However, the use of a rheostat control is imprecise. Moreover, even if the rheostat setting may be reproduced, a given amount of current through the thermoelectric device does not necessarily lead to the same temperature of the probe tip, since the temperature may vary depending upon whether the probe tip is in contact with the tooth to be tested or not. Overheating of the probe tip, in addition to possibly subjecting the instrument to damage, represents a source of potential injury to a patient. Nevertheless, the use of a thermoelectric device to provide extremes of temperature represents a substantial improvement over older methods which involved placing ice or volatile liquids directly on the tooth.
With respect to electrical stimulation, it is known to provide a test device having a probe tip to which a controlled high voltage is intermittently applied. Moreover, it is known to provide such an instrument with circuitry for automatically increasing over time the voltage level that is applied to the tip, and circuitry for checking electrical contact between the probe tip and the tooth and resetting the voltage to its lowest level if electrical contact is interrupted for more than a predetermined time.
The use of a controlled voltage for providing localized stimulation is not without its disadvantages. In particular, in the event that a non-vital tooth to be tested is not properly dried, the saliva may provide electrical paths to other vital teeth, thus providing a false indication of vitality. Moreover, if the contact between the probe tip and a vital tooth to be tested is poor, but not sufficiently poor to cause the contact checking circuitry to provide an indication, the voltage actually applied to the tooth is smaller than believed, and a false indication of non-vitality results.